Published online 9 January 2009 | Nature | doi:10.1038/news.2009.3

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A waterway for one-way waves

There's no way back in a row of water-powered see-saws.

wave machineSee the marvellous, unidirectional wave machine in action.J. Lindner/College of Wooster

Waves that can travel in only one direction have been created using apparatus bought at a local hardware store.

In the inexpensive device rigged up by John Lindner of the College of Wooster in Ohio and his colleagues, trays suspended like see-saws from a central axle are tipped one way or another by jets of water. The trays are linked together in a row by levers in such a way that a wave of tipping motions can travel in only one direction along the array (see video).

Such one-way waves have been predicted theoretically, and researchers have demonstrated the principle in electronic circuits. But Lindner and his colleagues wanted to create a one-way wave in a mechanical system, visible to the naked eye.

It turned out to be surprisingly easy, using a bit of engineering ingenuity and materials that they found at a home-improvement store. "It can be inexpensively built, even by a home hobbyist," says Lindner. "It's not the Large Hadron Collider."

"An experiment like this may not look impressive to someone designing a synchrotron, but it's not the kind of thing someone can put together in a couple of afternoons," says Andy Ruina, a specialist in biomechanics at Cornell University in Ithaca, New York. "If it works, it is impressive."

Mechanical marvel

The row of tipping trays is like a string of beads, where a shake at one end will send a wave rippling along it. Normally, a shake in the middle of the string would send waves travelling outwards in both directions. But if a bead is linked in such a way that it can only transfer motion to the bead on its right, for example, the waves must travel in that direction alone.

Lindner's device, described in Physical Review E1, powers the waves with water flowing from rubber tubes suspended above the trays. A tray, filling with water, eventually tilts to one side. This moves a lever that redirects the water tube of the neighbouring tray, causing it to fill on the opposite side.

Each time a tray is pushed one way, the next one in line — in a predetermined direction — is pushed the other way. The last in line is coupled back to the first through a series of gears and axles, so that the trays act as though arranged in a ring, allowing waves to propagate endlessly along the row.

Such robust, focused waves, called solitons, are found in other systems both in nature and in technology: water solitons may propagate in rivers and canals, and light solitons are used in optical-fibre telecommunications.

It's not yet clear that the apparatus offers any insights into these other processes. But it may provide an analogue for unidirectional supersonic sound waves, Lindner suggests, or even for the propagation of waves through anisotropic materials, where a wave's speed differs in different directions.

The team is now considering how to build a mechanical array in which unidirectional wave motion can occur in two dimensions. "We have designed but not yet built a two-dimensional hydromechanical array of one-way coupled oscillators," says Lindner, who also hopes to create a similar array using light-emitting diodes. 

  • References

    1. Lindner, J. F., Patton, K. M., Odenthal, P. M., Gallagher, J. C. & Breen, B. J. Phys. Rev. E 78, 066604 (2008).
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